Temporary staff and VerticalAlignment

[Roman Stawski]

Hello people

I've come across a problem in 2.15.24 which I don't /believe/ existed in earlier
2.15 versions (although I haven't checked and I couldn't say when it appeared)

---
    \version "2.15.24"

    \new ChoirStaff \with {
        \consists Span_bar_engraver
    } {
        \new Staff = main {
            <<
                \relative c'' { c1 c1 c1 }
                \\ {
                    s1
                    \new Staff \relative c'' { g1 g1 }
                }
            >>
        }
    }
---

As this stand this generates a programming error "grob does not belong to a
VerticalAlignment". The output corresponds to what I want with the bar lines
spanning both staves.

However, if I comment out the \consists line no error appears. (Of course the
output isn't the same either -- the bar lines are confined to their own staves).

Does anyone have an idea how to get an error free compile and the bar lines
spanning.

Thanks for any help